Method for preparing glass substrates for magnetic recording mediums

ABSTRACT

A method for preparing a glass substrate for magnetic recording mediums comprises the steps of lapping a glass substrate, then chemically treating the glass substrate for strengthening to form a strengthening layer having a thickness of not less than 20 μm and polishing the chemically strengthened glass substrate so that not less than 10 μm of the strengthening layer on the main face is removed, while leaving not less than 10 μm of the strengthening layer on the main surface of the glass substrate. The method for preparing a glass substrate for magnetic recording mediums permits the easy preparation of such glass substrates having excellent flatness and excellent surface smoothness and also permits the marked improvement of the rate of machine operation and production efficiency.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for preparing a glass substrate for magnetic recording mediums and more specifically to a method which permits the production of a glass substrate, in a high efficiency, which is excellent in flatness.

[0003] 2. Description of the Prior Art

[0004] To improve the recording density of a recording medium, there has been tried to reduce the flying height (i.e., the distance between a magnetic head and the surface of a recording medium) as the capacity of magnetic disc recording devices has increasingly been high and this in turn requires the development of a substrate for magnetic recording mediums having excellent surface smoothness and a smaller amount of warp, i.e., excellent in flatness.

[0005] There have conventionally been used aluminum alloys and glass as substrate materials for magnetic recording mediums of hard disks. Since the glass may easily be polished to a high surface smoothness as compared with the aluminum alloys and therefore, it would be a material favorable for the production of substrates each having a low flying height. However, it is in general fragile. For this reason, the glass only has poor practicability unless it is subjected to any treatment for strength in advance and accordingly, it has been practically used after subjecting it to a surface chemical treatment for strengthening.

[0006] In general, the glass substrate for magnetic recording mediums has been produced according to the following production steps in order:

[0007] (a) a step for forming a raw glass material into a disk-like shape;

[0008] (b) a step for chamfering the inner and outer peripheral portions of the disk-like glass product;

[0009] (c) a step for lapping the glass product to a predetermined plate thickness;

[0010] (d) a polishing step for making the surface of the glass product smooth by grinding; and

[0011] (e) a step for chemically strengthening the surface thereof.

[0012] In this respect, the polishing step (d) in general comprises a first polishing stage for eliminating a deteriorated layer formed due to processing, such as cracks formed on the glass substrate during the step (a) for forming a disk-like product and the step (c) for lapping and a second lapping stage for smoothing the surface of the glass substrate to a predetermined flatness.

[0013] In addition, the step (e) for chemically strengthening the surface of the glass substrate is carried out by exchanging ions present on the glass surface layer for desired ions, but there has been known that the ion-exchange treatment would increase the unevenness of the surface of the resulting glass substrate as compared with that observed prior to the treatment. Moreover, the additional polishing of the glass substrate which has been chemically treated for strengthening can make the surface of the glass substrate smoother, but it has been believed that this additional polishing step has a tendency to increase the amount of warp of the glass substrate finally obtained.

[0014] Up to now, there have been proposed a variety of methods for eliminating the foregoing drawbacks, such as a method for preparing a glass substrate for magnetic recording mediums (Japanese Examined Patent Publication (hereinafter referred to as “J.P. KOKOKU”) No. Hei 3-52130) which comprises a chemically strengthening treatment carried out after a first and second stages for polishing; a method for preparing a glass substrate for magnetic recording mediums (Japanese Un-Examined Patent Publication (hereinafter referred to as “J.P. KOKAI”) Nos. Hei 7-134823 and Hei 8-124153), which comprises the steps of a chemical treatment for strengthening after a first polishing stage; and a second polishing stage after the chemical treatment for strengthening, i.e., a stage for polishing the glass substrate to a lowest possible extent (to a thickness of not more than 1 μm) in order to eliminate the unevenness of the surface of the glass substrate generated by the chemical strengthening treatment.

[0015] When the chemical strengthening treatment is carried out after the first or second polishing stage, however, the glass substrate which has not yet been chemically treated for strengthening is subjected to a strong abrasive-resistant polishing action and this may increase the probability of causing cracks on the glass substrate during polishing because the glass has brittleness by nature. This in turn leads to not only the reduction in production yield, but also a very low percent of machine operation due to the requirement of a step for cleaning the polishing machine for removing the cracked glass pieces.

[0016] In addition, the chemical strengthening treatment would reduce the flatness of the glass substrate as compared with that observed prior to the chemical treatment and therefore, the polishing operations should be carried out while allowing, in advance, the reduction of flatness due to the subsequent chemical strengthening treatment.

[0017] If the chemical strengthening treatment is performed after the second polishing stage, the flatness of the glass substrate is reduced since unevenness of the surface after the chemical treatment is on the order of several tens of nanometers.

SUMMARY OF THE INVENTION

[0018] Accordingly, it is an object of the present invention to provide a method for preparing a glass substrate for magnetic recording mediums, which permits the production of a glass substrate having excellent flatness using the same polishing machine used in the conventional methods and which can efficiently produce a glass substrate while reducing the amount of cracks formed to a lowest possible level and preventing the reduction of the percent of machine operation due to, for instance, the requirement of a step for cleaning the polishing machine.

[0019] The inventors of this invention have conducted various studies to accomplish the foregoing object, have found that, in a method for preparing a glass substrate for magnetic recording mediums, the foregoing object can effectively be accomplished by chemically treating a glass substrate for strengthening after subjecting it to lapping so that a thick strengthening layer is formed on the surface and then polishing the strengthened glass substrate to remove the strengthening layer on the main surface of the substrate to a greater depth of the layer and thus have completed the present invention.

[0020] The present invention thus relates to a method for preparing a glass substrate for magnetic recording mediums, which comprises the steps of lapping a glass substrate, then chemically treating the glass substrate for strengthening to form a strengthening layer having a thickness of not less than 20 μm and polishing the strengthened glass substrate so that not less than 10 μm of the strengthening layer is removed, while leaving not less than 10 μm of the strengthening layer on the main surface of the glass substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The method of the present invention will hereinafter be described in more detail with reference to the following non-limitative preferred embodiments.

[0022] In the present invention, the lapping process means a step for processing a glass substrate to a predetermined plate thickness and preferably comprises two stages, i.e., a first lapping stage for grinding the glass substrate to a predetermined thickness and a second lapping stage as a preliminary step for the subsequent polishing step. In the lapping step, well known abrasive materials may be used and specific examples thereof include abrasive grains of silicon carbide, alumina, colloidal silica and diamond. In this respect, the average particle size of the abrasive material desirably ranges from about 10 to 30 μm for the first lapping stage and about 5 to 15 μm for the second lapping stage.

[0023] In the present invention, the term “chemically strengthening treatment” means a treatment for exchanging ions existing in the proximity to the glass surface for those having a higher ionic radius in a temperature range of not more than the glass transition temperature of the glass used to thus establish a compression stress in the glass surface region. For instance, this chemical strengthening treatment can be carried out by immersing a glass substrate in a mixed fused salt of potassium nitrate and sodium nitrate to thus exchange lithium ions present in the glass substrate for potassium and sodium ions present in the fused salt.

[0024] In the method of the present invention, the glass material used is not restricted to any specific one inasmuch as they have an ability to form a strengthening layer by such a chemical strengthening treatment and specific examples thereof include aluminosilicate glass materials, soda-lime glass materials, borosilicate glass materials and aluminoboro-silicate glass materials. Particularly preferred are, for instance, glass materials capable of easily forming a thick strengthening layer such as aluminosilicate glass materials which comprises, for instance, 55 to 77% by weight of SiO₂, 3 to 25% by weight of Al₂O₃, 3 to 10% by weight of Li₂O and 3 to 13% by weight of Na₂O as well as ZrO₂, MgO, ZnO, B₂O₃ or the like as optional components.

[0025] In the present invention, the chemical strengthening treatment is carried out to such an extent that the strengthening layer is formed on each plane of the glass substrate in a thickness of not less than 20 μm, preferably not less than 30 μm and more preferably not less than 50 μm. This is because if the thickness of the strengthening layer is less than 20 μm, the strengthening layer is completely removed or it is too thin to ensure the desired level of flatness in the subsequent polishing step.

[0026] In the present invention, the “polishing step” means a step for polishing a glass substrate to thus make the surface of the substrate smooth and to prevent the generation of any warp and cerium oxide is, for instance, used as an abrasive material. In the method for preparing a glass substrate for magnetic recording mediums according to the present invention, a thick strengthening layer has been formed on the surface of the glass substrate prior to the polishing step and accordingly, there is almost no danger of damaging the glass substrate during the polishing step. Moreover, the unevenness (on the order of several tens of nanometers) formed on the surface of the glass substrate by the chemical strengthening treatment can completely be eliminated by the subsequent polishing step and thus the surface of the resulting glass substrate would have considerably excellent flatness. In addition, the finally obtained glass substrate does not cause any warp even if both of the principal faces of the substrate are polished to the possible uniformity by a conventional polishing machine and the strengthening layer is removed to a great depth by polishing, since the remaining strengthening layer is still sufficiently thick. To this end, it is necessary to remove the strengthening layer on each principal face in a thickness of not less than 10 μm and preferably not less than 15 μm, in the polishing step of the method according to the present invention, while after the polishing step, the thickness of the remaining strengthening layer should be not less than 10 μm and preferably not less than 15 μm.

[0027] The method of the present invention will hereinafter be described in more detail with reference to the following non-limitative working Examples and Comparative Examples. In the following Examples and Comparative Examples, the thickness of the strengthening layer is determined through observation by a polarization microscope and the flatness of the glass substrate is determined by a light wave interference system using a He—Ne laser. More specifically, the flatness achieved after lapping is determined by FT-11 available from NIDEK Corporation, while that observed after polishing is determined by KIF-401 available from Olympus Optical Co., Ltd.

EXAMPLE 1 AND COMPARATIVE EXAMPLE 1

[0028] A plurality of doughnut-shaped glass substrates each having an outer diameter of 65 mm, an inner diameter of 20 mm and a thickness of 0.68 mm were produced from an aluminosilicate glass plate which comprised 63.0% by weight of SiO₂, 12.0% by weight of Al₂O₃, 4.5% by weight of Li₂O, 10.0% by weight of Na₂O, 3.5% by weight of ZrO₂, 2.0% by weight of MgO, 2.0% by weight of ZnO and 3.0% by weight of B₂O₃, by a processing step for achieving desired inner and outer diameters and a lapping step, which were performed according to the procedures for producing substrates for the usual hard disk. Thereafter, these substrates were subjected to a chemical strengthening treatment by immersing them in a mixed fused salt of potassium nitrate (70% by weight) and sodium nitrate (30% by weight) maintained at 380° C. for one hour so as to form a strengthening layer having a thickness of 60 μm.

[0029] Then 100 substrates thus chemically strengthened were fitted to an abrasive machine and subjected to a polishing treatment using cerium oxide slurry as an abrasive material and a polishing pad till the thickness of the substrate removed by the polishing reached 20 μm for each face This polishing operation was repeated seven times (700 substrates in all), the number of substrates which had to be rejected as defectives such as those cracked, broken and/or fractured was determined for each polishing operation and there were observed the results as listed in Table 1 given below.

[0030] On the other hand, the doughnut-shaped substrate free of any chemical strengthening treatment were likewise subjected to a polishing treatment by the same procedures used above and the polishing operation was also repeated 7 times, as Comparative Example. The number of comparative substrates which had to be rejected as defectives such as those cracked, broken and/or fractured was determined for each polishing operation and there were observed the results as listed in Table 1 given below. TABLE 1 Number of Substrates to be rejected as defectives Polishing Example 1 Comp. Ex. 1 Operation Fracture Breakage Crack Fracture Breakage Crack 1st 0 0 0 0 1 0 2nd 0 0 1 0 2 3 3rd 0 0 0 0 0 1 4th 0 0 0 1 2 3 5th 0 0 1 0 0 0 6th 0 0 0 0 0 2 7th 0 0 0 1 3 4 Total* 0 0 2 2 8 13 

EXAMPLE 2 AND COMPARATIVE EXAMPLE 2

[0031] A plurality of doughnut-shaped glass substrates each having an outer diameter of 65 mm, an inner diameter of 20 mm and a thickness of 0.68 mm were produced from the aluminosilicate glass plate identical to that used in Example 1, by a processing step for obtaining desired inner and outer diameters and a lapping step, which were carried out according to the procedures for producing substrates for the usual hard disk. These substrates were inspected for the flatness, followed by selection of substrates which met the standard requirements for flatness, i.e., those having a flatness of not more than 5 μm and a chemical strengthening treatment of these substrates by immersing them in a mixed fused salt of potassium nitrate (70% by weight) and sodium nitrate (30% by weight) maintained at 380° C. for one hour so as to form a strengthening layer having a thickness of 60 μm.

[0032] Then 100 substrates thus chemically strengthened were fitted to an abrasive machine and subjected to a polishing treatment using cerium oxide slurry as an abrasive material and a polishing pad till the thickness of the substrate removed by the polishing reached 20 μm for each face. Five substrates were arbitrarily selected out of these 100 substrates and inspected for their flatness. The results of flatness-determination before the chemical strengthening treatment (before C.S.T.) and after the polishing step (after P.S.) are summarized in the following Table 2.

[0033] On the other hand, substrates, as comparative samples, were prepared by subjecting to the foregoing chemical strengthening treatment and the polishing step in the reverse order or substrates were produced by first polishing and then chemically strengthening, followed by picking out 5 substrates out of these comparative samples and determination of the flatness thereof. The results of flatness-determination before the polishing step (before P.S.) and after the chemical strengthening treatment (after C.S.T.) are also summarized in the following Table 2. TABLE 2 (Flatness (μm)) Sample Example 2 Comp. Ex. 2 No. Before C.S.T. After P.S. Before P.S. After C.S.T. 1 3.6 1.2 4.2 3.9 2 2.4 0.6 4.5 3.9 3 4.2 0.9 2.4 2.4 4 2.4 1.2 2.7 2.7 5 4.2 1.5 2.4 2.1 Average 3.4 1.1 3.2 3.0

[0034] The averaged surface roughness R_(a) of these 5 samples of Example 2 was found to be 0.25 nm and the averaged maximum surface roughness R_(max) for these samples was found to be 1.8 nm. On the other hand, the averaged surface roughness R_(a) of the 5 samples of Comparative Example 2 was found to be 0.40 nm and the averaged maximum surface roughness R_(max) for these samples was found to be 2.5 nm.

[0035] The foregoing results clearly indicate that the glass substrate obtained by carrying out the chemical strengthening treatment prior to the polishing step has excellent flatness and good surface smoothness and can ensure a high production yield as compared with the substrate prepared by changing the order of the foregoing steps even if these substrates are completely identical to each other till they are subjected to the lapping process.

EXAMPLE 3 AND COMPARATIVE EXAMPLE 3

[0036] The same procedures used in Example 2 were repeated except for picking out substrates which did not meet the standard requirements for flatness, i.e., those having a flatness of more than 5 μm out of those subjected to lapping and using them for the subsequent processing steps to give glass substrates and then these substrates were inspected for their flatness (Example 3). The results of flatness-determination before the chemical strengthening treatment (before C.S.T.) and after the polishing step (after P.S.) are summarized in the following Table 3.

[0037] Separately, the same procedures used in Comparative Example 2 were repeated except for picking out substrates which did not meet the standard requirements for flatness, i.e., those having a flatness of not more than 5 μm out of those subjected to lapping and using them for the subsequent processing steps to give glass substrates and then these substrates were inspected for their flatness (Comparative Example 3). The results of flatness-determination before the polishing step (before P.S.) and after the chemical strengthening treatment (after C.S.T.) are also summarized in the following Table 3. TABLE 3 (Flatness (μm)) Sample Example 3 Comp. Ex. 3 No. Before C.S.T. After P.S. Before P.S. After C.S.T. 1 6.3 3.3 8.4 6.6 2 6.0 3.0 6.6 6.3 3 8.1 4.2 8.1 7.2 4 7.5 2.7 6.0 5.7 5 6.0 2.4 6.3 5.4 Average 6.8 3.1 7.1 6.2

[0038] As will be seen from the data listed in Table 3, the method of the present invention permits the production of glass substrates for magnetic recording mediums having flatness which meets the standard requirements even when using substrates which are off standards, i.e., whose flatness at the stage after the lapping step exceeds 5 μm.

EXAMPLE 4

[0039] Substrates were prepared by repeating the same procedures used in Example 3 except for picking out substrates which did not meet the standard requirements for flatness, i.e., those having a flatness observed after the lapping step of more than 5 μm and except for forming strengthening layers having thicknesses of 40, 80 and 120 μm, in addition to a thickness of 60 μm and then these substrates were inspected for their flatness. The results of flatness-determination before the chemical strengthening treatment (before C.S.T.) and after the polishing step (after P.S.) are summarized in the following Table 4. TABLE 4-1 (Flatness (μm)) Thickness of Strengthening Layer Sample 40 μm 60 μm No. Before C.S.T. After P.S. Before C.S.T. After P.S. 1 6.0 3.0 6.3 3.3 2 5.7 5.1 6.0 3.0 3 5.7 5.4 8.1 4.2 4 6.3 4.8 7.5 2.7 5 8.1 5.1 6.0 2.4 Average 6.4 4.7 6.8 3.1

[0040] TABLE 4-2 (Flatness (μm)) Thickness of Strengthening Layer Sample 80 μm 120 μm No. Before C.S.T. After P.S. Before C.S.T. After P.S. 1 8.4 4.2 6.3 1.8 2 5.7 3.3 6.0 1.2 3 6.3 2.4 5.7 1.2 4 6.0 2.1 7.5 1.8 5 6.3 2.1 6.0 0.9 Average 6.5 2.8 6.3 1.4

[0041] As will be seen from the data listed in Table 4, the greater the thickness of the strengthening layer, the higher the flatness of the resulting glass substrate.

[0042] As has been discussed above in detail, the method for preparing a glass substrate for magnetic recording mediums according to the present invention permits the easy preparation of such glass substrates having excellent flatness and excellent surface smoothness and also permits the marked improvement of the rate of machine operation and production efficiency. 

What is claimed is:
 1. A method for preparing a glass substrate for magnetic recording mediums, consisting essentially of the steps of lapping a glass substrate, then chemically treating the glass substrate for strengthening to form a strengthening layer having a thickness of not less than 20 μm and polishing the strengthened glass substrate so that not less than 10 μm of the strengthening layer on the main face is removed, while leaving not less than 10 μm of the strengthening layer on the main surface of the glass substrate.
 2. The method of claim 1 wherein the thickness of the strengthening layer is not less than 30 μm and wherein not less than 15 μm of the strengthening layer on the main face is removed, while leaving not less than 15 μm of the strengthening layer on the main surface of the glass substrate.
 3. The method of claim 1 wherein the lapping step is a two-stage lapping step comprising first and second lapping stages.
 4. The method of claim 3 wherein an abrasive material used in the lapping step has a particle size ranging from 10 to 30 μm for the first lapping stage and 5 to 15 μm for the second lapping stage.
 5. The method of claim 1 wherein the chemically strengthening treatment comprising immersing the glass substrate in a mixed and fused salt.
 6. The method of claim 1 wherein the glass substrate is an aluminosilicate glass material. 